1. Field of the Invention
The present invention relates to a method and system for achieving superior performance with a multi-cell battery which would typically be used with either a purely electric vehicle (EV), or a hybrid electric vehicle (HEV), or a fuel cell vehicle (FCV). The present method and system enhance battery performance by providing superior cooling, without the need for upsized cooling devices.
2. Disclosure Information
Traction batteries used in various EVs, HEVs. And FCVs are typically of the nickel metal hydride or lithium-ion construction. Such batteries offer the advantage of high energy density and the ability to rapidly recharge. Unfortunately, lithium-ion and nickel metal hydride batteries need careful temperature management to assure high performance and longer life with high efficiency. Because automotive vehicles must operate under ambient temperature extremes, temperature management presents problems. More specifically, the self-heating of battery cells due to parasitic energy loss when the batteries are being discharged or recharged must be managed carefully if the temperature is to be kept below the battery's maximum operational range. Moreover, uneven temperature conditions across a battery pack may cause individual changes in the electrochemical reaction characteristics of individual cells, which can affect the voltage and internal resistance of each cell, as well as its life. Although a battery system controller will attempt to regulate battery reaction uniformly between battery cells through balancing, the cell voltage across the battery system, an uneven temperature profile can cause non-homogeneous battery electric chemical reaction rates across the cell stack and result in accumulative degradation of battery cells. Moreover, it is known that excessive temperature will damage battery cells due to hyperactive chemical reaction rates.
Conventional battery cooling systems such as that disclosed in U.S. Pat. No. 6,407,533, use uni-directional coolant flow for battery cooling. In a uni-directional flow system, air, or some other heat transfer fluid, is introduced in at one boundary of a battery pack and discharged from the opposite boundary. The intake and discharge ports do not change function with time. In this sort of cooling arrangement, the greatest temperature differential, or Δ T, between battery cells occurs between the inlet and outlet ends of the cooling path through the battery. Although the system of the '533 patent includes seasonal switching of airflow direction, no reciprocation occurs during normal operation in either the heating or cooling modes. As a result, the system of the '533 patent suffers from the deficiencies noted above.
The present inventors have determined that a battery temperature management system including the use of reciprocating or bi-directional heat transfer fluid flow for battery cooling and heating, in which the fluid flow is reversed after a predetermined time period will cause the time averaged temperature of the battery cells at both ends of the battery pack to be equal. And, if the period between successive flow reversals is optimized, the differential temperature between any two successive cells in the battery stack may be minimized to a very great extent. This will cause enhanced performance and life and efficiency of the battery stack because of the balanced usage of the battery cells.
Those skilled in the art will appreciate the profound utility of a system according to the present invention, particularly in the view of the fact that a temperature rise of only 10° C. will normally double the battery reaction rate. This is undesirable because higher rates at higher temperatures increase the rate of deterioration of the battery and may cause premature damage of battery cells and failure of the battery system itself.